Twitter has been rolling out the new retweet feature to some users. If you're one of them, you'll have a new button saying "Retweet" below each tweet, and also have notifications about who retweeted you (using this new feature, I'd guess).
A lot of people are already complaining that the use of the feature is too limited, mainly because it doesn't allow you to add a comment to a retweet by putting some text before the actual retweet like this:
Actually, I think the critics are right, the comments are a useful feature, and hopefully, twitter will add it rather quickly.
One of the nice things about the retweet feature is that you can actually see who has been retweeting you. But even when you're not one of the beta testers, you can get that information pretty easily from twimpact. Just type in your name in the search bar, or go directly to http://twimpact.com/user/yourname, and you will have a list of all your retweeted messaged with a list of who retweeted you, at least back till June 2009.
For example, here is my (pretty pathetic) list:
And before you complain, we know that not all retweets make it in there, but this is mostly due to the fact that twitter's search infrastructure is eventually consistent, at best.